Interferometric angle monitor

Apparatus for measuring the angular displacement of a rotatable object having means for reflecting optical beams, by measuring the linear displacement of a series of optical interference fringes, comprising:

• a source (32) emitting monochromatic, coherent radiation (34);
• beam expanding optics (36) disposed relative to said source to focus said radiation;
• a first aperture (38) positioned at the focus of said beam expanding optics to pass said radiation as a single diverging spherical beam (40);
• means including a plurality of reflective surfaces (42, 48, 52, 56/60, 62, 68, 74) for dividing said single spherical beam and projecting toward said reflective means (20) a pair of generally parallel, diverging spherical beams (b.sub.1, b.sub.2) emanating from a pair of virtual sources (S.sub.1, S.sub.2) separated by a distance (d);
• an imaging optical system (10) having an optical axis intersecting said object, positioned to focus radiation from said pair of diverging spherical beams reflected from said reflective means upon a focal plane (12) optically at a distance (Z) from said reflecting means of a significantly larger magnitude in relation to said distance (d) separating said virtual sources whereby an interference fringe pattern (14) is formed at said focal plane, and wherein said pattern translates linearly across said focal plane in response to angular movement of said reflective means;
• a second aperture (26) positioned behind said focal plane and centered upon said optical axis of said imaging system;
• and
• a photodetector exposed to said radiation via said second aperture for providing a signal indicative of the intensity of said fringe pattern at said second aperture.